(i) Field of the Invention:
This invention relates to finely powdered polyvalent metal salts of 2-mercaptopyridine-N-oxide and to a method for producing same. It also relates to antibacterial compositions comprising such metal salts.
(ii) Description of the Prior Art:
Polyvalent metal salts of 2-mercaptopyridine-N-oxide (which may be hereinafter referred to simply as polyvalent metal salts) are known to be effective as antibacterial agents and are widely used not only as ordinary bactericides, but also as dandruff removers being added to shampoo, rinse and the like. The polyvalent metals of the salts are, for example, calcium, magnesium, barium, strontium, zinc, cadmium, tin, zirconium and the like, of which zinc is widely used.
However, these polyvalent metal salts, e.g. zinc salt has a solubility in water of 15 ppm at 25.degree. C. and is thus sparingly soluble in water with a high specific gravity, so that they are sold and supplied in the form of a powder or an aqueous dispersion. Because of the high specific gravity, the metal salts have the disadvantage that even when suspended or dispersed in solvent, they are liable to coagulate, settle and separate.
Several shampoos and rinses containing the polyvalent metal salts are put on the market. In order to stably incorporate the salts into shampoo or rinse compositions, it is unavoidable to make them highly viscous or to add specific types of polymers or clay minerals. This leads to the disadvantage that the compositions become poor in performance, e.g. with shampoos, the foaming ability becomes poor with objectionable feeling to the touch after washing of the hair.
Recently, several methods of stably keeping such metal salts in compositions have been proposed including a method using the salts in combination with hydroxyalkyl celluloses (Japanese laid-open Application No. 57-80499), a method using the salts in combination with amphoteric polymer compounds (Japanese Laid-open Application No. 54-41909), a method in which HPC is used in combination (Japanese Laid-open Application No. 35-97010), and a method in which polyacrylic acid/polyacrylates are used in combination.
However, all these methods still make use of the specific types of polymers and thus problems on the rise of cost and the performance of the products are not solved. Moreover, these methods are all techniques of applications to shampoos. The possibility of applications to other types of products are not made clear.